The G650 is the replacement or step up from the G450 and G550... Think you are getting wires crossed a little..
The G650 is not a replacement for the G450 and G550. The G650 is an expansion of the Gulfstream model line.
The G550 did not replace the G450 -- the G550 is a larger, longer range, and more expensive aircraft. The same is true for the G650 -- it is a larger, longer range, and more expensive aircraft than the G550.